python not able to convert very small decimal to log

Question

I am using a function that multiplies probabilities there by creating very small values. I am using decimal.Decimal module to handle it and then when the compuation is complete I convert that decimal to logofOdds using math.log module/function. But, below a certain proability python cannot convert these very small probabilities to log2 or 10 of likelyhood ratio.

I am getting ValueError: math domain error

So, I printed the value before the traceback started and it seems to be this number:

2.4876626750969332485460767406646530276378975654773588506772125620858727319570054153525540357327805722211631386444621446226193195409521079089382667946955357511114536197822067973513019098983691433561051610219726750413489309980667312714519374641433925197450250314924925500181809328656811236486523523785835600132361529950090E-366

Other small numbers like this are getting handled by math.log though in the same program: 5.0495856951184114023890172277484001329118412629157526209503867218204386939259819037402424581363918720565886924655927609161379229574865468595907661385853201472751861413845827437245978577896538019445515183910587509474989069747817303700894727201121392323641965506674606552182934813779310061601566189062725979740753305935661E-31

Is it true? any way to fix this. I know I can take the log of the probs and then sum it along the way, but when I tried to do that, it seems I have to update several places in my program - could take significant hours or days. and there is another process to convert it back to decimal.

Thanks,

Answer 1

If you want to take logarithms of Decimal objects, use the ln or log10 methods. Aside from a weird special case for huge ints, math.log casts inputs to float.

whatever_decimal.ln()